Esters of cyclohexane polycarboxylic acids as plasticizers in rubber compounds

ABSTRACT

It has been discovered that the use of a plasticizer, an ester of a cyclohexanepolycarboxylic acid, especially a 1,2-cyclohexanedicarboxylic acid ester, that is environmentally non-toxic, can replace environmentally unfriendly phthalate plasticizers, such as dioctyl phthalates used in vulcanizable rubber compositions, especially for tires, on a one to one weight basis (phr) without sacrificing beneficial rubber properties.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/556,767 filed Sep. 10, 2009, which claims the benefit of priorityfrom U.S. Provisional Application Ser. No. 61/095,799 filed Sep. 10,2008. The prior applications are hereby incorporated into the presentapplication by reference for all purposes.

BACKGROUND OF THE INVENTION

Diesters of phthalic acid, particularly dioctyl phthalates (DOP) and thelike, are widely used as plasticizers for the processing of many PVCsoft plastics, and in the rubber, paint and emulsifier industries, whereit can improve the rebound elasticity of products and reduce permanentdistortions. In tire treads, in particular, phthalate diesterplasticizers can improve snow traction performance. However, due toconcerns over the toxicological effects of phthalates, environmental andhealth groups are now pushing to restrict the use of phthalates,especially in products with direct human contact, such as cosmetics,toys, medical devices and the like.

Although the rubber industry has not yet been directly involved in suchenvironmental concerns, it has been considered prudent by this companyto investigate substitutions for phthalate diesters, such as DOP and thelike, in rubber compositions. Several substitute compounds have beeninvestigated; however, a one-to-one quantitative substitution of any ofthese compounds for DOP, that provided the same beneficial rubberproperties as DOP, has not been found previously.

Recently, it has been discovered that certain esters ofcyclohexanepolycarboxlyic acids are environmentally non-toxic and can beused as plasticizers for polyvinyl chloride to enable products withcomparable mechanical properties to be obtained using less polyvinylchloride. Further, esters and other derivatives ofcyclohexanepolycarboxlyic acids have been shown to be useful ascomponents of paints, varnish, inks, adhesives, sealants and the like;as components in lubricating oil or cutting oil, especially in metalworking; and as plasticizers in the production of semi-rigid to highlyflexible materials, such as medical materials, particularly blood bags,tubing and the like, and cling film for food applications, and the like.

However, the use of such cyclohexanepolycarboxylic acid esters asplasticizers in rubber compounds has not been reported. Nor have theeffects of such chemicals on rubber properties been reported. In view ofour previously unsuccessful search for non-toxic plasticizers that couldbe used in quantitative amounts similar to DOP and produce equivalentdesirable rubber properties, expectations were not high thatcyclohexanepolycarboxylic acid esters would be a suitable substitute fordiesters of phthalates, such as DOP and the like, in rubber compounds.

SUMMARY OF THE INVENTION

Unexpectedly, it has been found that non-phthalate plasticizers, such ascertain cyclohexanepolycarboxylic acid esters and, particularly,1,2-cyclohexanedicarboxylic acid esters and the like, are useful asplasticizers in rubber compounds. Further, such1,2-cyclohexanedicarboxylic acid esters can be used quantitatively as aone-to-one replacement for diesters of phthalic acid and they provideequivalent rubber properties to those provided by the phthalic aciddiesters, including desirable wet traction and snow traction, whilemaintaining other advantageous physical properties of the rubbers.Therefore, esters of cyclohexanepolycarboxylic acids, especially1,2-cyclohexanedicarboxylic acid esters, are acceptable and desirableenvironmentally non-toxic replacements for phthalic acid diesters inrubber compounds and tire components made from them, especially tiretreads.

In particular, the invention is concerned with a vulcanizable rubbercomposition comprising an elastomer; a reinforcing filler comprising aselection from the group consisting of silica, carbon black, andmixtures thereof; a plasticizer comprising a 1,2-cyclohexanedicarboxylicacid ester; and a cure agent. When the reinforcing filler comprisessilica, the composition further comprises a selection from the groupconsisting of a silica coupling agent, a silica shielding agent, andmixtures thereof, as described below.

The 1,2-cyclohexanedicarboxylic acid ester plasticizer is present in thecomposition as a replacement for a phthalic acid ester plasticizer suchas, but not limited to, dioctyl phthalate and the like, often used inrubber compositions. Therefore, the invention composition issubstantially free of a plasticizer comprising a phthalic acid esterand, in a very suitable arrangement, the 1,2-cyclohexanedicarboxylicacid ester is present in the composition in an amount sufficient tosubstantially replace an equivalent amount by weight of a phthalic acidester plasticizer that could be otherwise used in the composition. As anon-limiting example, the 1,2-diisononylcyclohexanedicarboxylic acidester can be employed on a weight basis per hundred parts rubber (phr)substantially as a one-to-one replacement for the phthalic acid esterplasticizer, dioctyl phthalate. The 1,2-cyclohexanedicarboxylic acidester can be present in the composition, for example, in an amount ofabout 0.01 to about 100 phr.

The elastomer in the vulcanizable rubber composition can be selectedfrom the group consisting of homopolymers of a conjugated diene monomer,and copolymers and terpolymers of the conjugated diene monomers withmonovinyl aromatic monomers and trienes and, in at least one arrangementof the invention, can be sulfur vulcanizable.

The invention also provides a pneumatic tire including at least onecomponent comprising a vulcanized rubber compound made from thevulcanizable rubber composition. In a very suitable arrangement, the atleast one component of the tire comprises a tire tread.

DETAILED DESCRIPTION OF THE INVENTION

The terms elastomer, polymer and rubber are used interchangeably herein,as is customary in the rubber industry.

It has been discovered unexpectedly that certaincyclohexanepolycarboxylic acid esters, such as1,2-cyclohexanedicarboxylic acid esters and the like, are useful asenvironmentally non-toxic plasticizers in rubber compounds and can besubstituted for esters of phthalates, such as dioctyl phthalate, onsubstantially a one-to-one weight basis per hundred parts rubber inrubber compositions, while maintaining essentially the same desirablerubber properties.

In one arrangement, the invention comprises a vulcanizable rubbercomposition comprising an elastomer; a reinforcing filler comprising aselection from the group consisting of silica, carbon black, andmixtures thereof; a plasticizer comprising a1,2-cyclohexane-dicarboxylic acid ester; and a cure agent. The1,2-cyclohexane-dicarboxylic acid ester can be, but is not intended tobe limited to, a 1,2-diisobutyl-cyclohexanedicarboxylic acid ester; a1,2-di-(2-ethylhexyl)-cyclohexane-dicarboxylic acid ester; a1,2-diisononylcyclohexane-dicarboxylic acid ester; a1,2-di-C₉-cyclohexane-dicarboxylic acid ester; a1,2-diisopentylcyclohexanedicarboxylic acid; a1,2-diisoheptyl-cyclohexanedicarboxylic acid; a1,2-diisodecylcyclohexane-dicarboxylic acid; a1,2-di-C₇₋₁₁-cyclohexanedicarboxylic acid; a1,2-di-C₉₋₁₁-cyclohexanedicarboxylic acid; a1,2-di-C₇₋₉-cyclohexanedicarboxylic acid; and mixtures thereof. In aparticularly suitable arrangement, the 1,2-cyclohexane-dicarboxylic acidester comprises a 1,2-diisononylcyclohexanedicarboxylic acid ester.

The 1,2-cyclohexanedicarboxylic acid ester can be present in thevulcanizable rubber composition in an amount of about 0.01 to about 100phr, suitably about 1 to about 40 phr, more suitably about 5 to about 20phr and, especially, about 5 to about 15 phr. The1,2-cyclohexanedicarboxylic acid ester can also be used as an oilextender for one or more elastomers employed in the composition. In thisarrangement, the 1,2-cyclohexanedicarboxylic acid ester can be used asan oil extender for the elastomer in the amount of about one to about 40parts, typically about 5 to about 40, of the total weight of the oilextended elastomer and this amount would be included in the total amountof the 1,2-cyclohexanedicarboxylic acid ester in the rubber composition.

The vulcanizable rubber composition is intended to be free ofenvironmentally toxic plasticizers and, therefore, is substantially freeof a plasticizer comprising a phthalic acid ester, such as dioctylphthalate and the like, that is often used as a plasticizer in suchrubber compositions. The 1,2-cyclohexanedicarboxylic acid ester ispresent in the composition in an amount sufficient to substantiallyreplace an equivalent amount by weight of a phthalic acid esterplasticizer, as a simple one-to-one replacement by weight (parts perhundred of rubber, phr).

When the reinforcing filler comprises silica, the vulcanizable rubbercomposition according to the invention further comprises a selectionfrom the group consisting of a silica coupling agent, a silica shieldingagent, and mixtures thereof. Such silica coupling agents and/orshielding agents are well known in the art of rubber compounding and aredescribed further below.

In another arrangement, the invention also provides a pneumatic tireincluding at least one component comprising a vulcanized rubber compoundmade from a vulcanizable rubber composition that comprises an elastomer;a reinforcing filler comprising a selection from the group consisting ofsilica, carbon black, and mixtures thereof; a plasticizer comprising1,2-cyclohexanedicarboxylic acid ester; and a cure agent. The1,2-cyclohexanedicarboxylic acid ester can comprise any of those listedabove. In a very suitable arrangement, the 1,2-cyclohexanedicarboxylicacid ester can comprise, but is not limited to,1,2-diisononylcyclohexanedicarboxylic acid. Although the vulcanizedrubber compound is used especially in tire treads, the compound can beused to make other components of tires, as described below.

The vulcanizable rubber compositions according to the invention cancomprise any solution polymerizable or emulsion polymerizable elastomer.Solution and emulsion polymerization techniques are well known to thoseof ordinary skill in the art. For example, conjugated diene monomers,monovinyl aromatic monomers, triene monomers, and the like, can beanionically polymerized to form conjugated diene polymers, or copolymersor terpolymers of conjugated diene monomers and monovinyl aromaticmonomers (e.g., styrene, alpha methyl styrene and the like) and trienemonomers. The elastomers that are typically employed within vulcanizablecompositions that are useful for making tires and tire componentsinclude both natural and synthetic elastomers. For example, theseelastomers include, without limitation, natural rubber, syntheticpolyisoprene rubber, styrene/butadiene rubber (SBR), polybutadiene,butyl rubber, neoprene, ethylene/propylene rubber, ethylene/propylenediene rubber (EPDM), acrylonitrile/butadiene rubber (NBR), siliconerubber, fluoroelastomers, ethylene acrylic rubber, ethylene vinylacetate copolymers (EVA) epichlorohydrin rubbers, chlorinatedpolyethylene rubber, chlorosulfonated polyethylene rubbers, hydrogenatednitrile rubber, tetrafluoroethylene/propylene rubber and the like.Preferred polymers for use in the vulcanizable rubber compositions ofthe invention include polyisoprene, polybutadiene, butadiene/isoprenecopolymer, butadiene/isoprene/styrene terpolymer, isoprene/styrenecopolymer, and styrene/butadiene copolymer. As used herein, the termelastomer or rubber will refer to a blend of synthetic and naturalrubber, a blend of various synthetic rubbers, or simply one type ofelastomer or rubber. When the preferred polymers are blended withconventional rubbers, the amounts can vary widely within a rangecomprising from about one to about 100 percent by weight of the totalrubber, with the conventional rubber or rubbers making up the balance ofthe total rubber (100 parts).

The elastomers that are useful in practicing this invention include anyof the various functionalized polymers that are conventionally employedin the art of making tires. For example, polymers can be terminallyfunctionalized, or functionalized throughout the polymer backbone, suchas with functional groups derived from an anionic polymerizationinitiator or a terminating or coupling agent. Preparation offunctionalized polymers is well known to those skilled in the art.Exemplary methods and agents for functionalization of polymers aredisclosed, for example, in U.S. Pat. Nos. 5,268,439, 5,496,940,5,521,309 and 5,066,729, the disclosures of which are herebyincorporated by reference. For example, compounds that provide terminalfunctionality that are reactive with the polymer bound carbon-lithiummoiety can be selected to provide a desired functional group. Examplesof such compounds are alcohols, substituted aldimines, substitutedketimines, Michler's ketone, 1,3-dimethyl-2-imidazolidinone, 1-alkylsubstituted pyrrolidinones, 1-aryl substituted pyrrolidonones, tintetrachloride, tributyl tin chloride, carbon dioxide, and mixturesthereof. Other useful terminating agents can include those of thestructural formula (R)_(a)ZX_(b), where Z is tin or silicon, R is analkyl having from about one to about 20 carbon atoms, a cycloalkylhaving from about 3 to about 30 carbon atoms; and aryl having from about6 to about 20 carbon atoms, or an aralkyl having from about 7 to about20 carbon atoms. For example, R can include methyl, ethyl, n-butyl,neophyl, phenyl, cyclohexyl, or the like. X is a halogen, such aschlorine or bromine, or alkoxy (—OR), “a” is an integer from zero to 3,and “b” is an integer from one to 4, where a+b=4. Examples of suchterminating agents include tin tetrachloride, tributyl tin chloride,butyl tin trichloride, butyl silicon trichloride, as well astetraethoxysilane, Si(OEt)₄, and methyl triphenoxysilane, MeSi(OPh)₃.The practice of the present invention is not limited solely to polymersterminated with these agents, since other compounds that are reactivewith the polymer bound carbon-lithium moiety can be selected to providea desired functional group.

The vulcanizable rubber compositions of the invention are preferablycompounded with reinforcing fillers, such as silica or carbon black, ora mixture of silica and carbon black. Examples of suitable silicareinforcing filler include, but are not limited to, precipitatedamorphous silica, wet silica (hydrated silicic acid), dry silica(anhydrous silicic acid), fumed silica, calcium silicate, and the like.Other suitable fillers include aluminum silicate, magnesium silicate,and the like. Among these, precipitated amorphous wet-process, hydratedsilicas are preferred. These silicas are so-called because they areproduced by a chemical reaction in water, from which they areprecipitated as ultrafine, spherical particles. These primary particlesstrongly associate into aggregates, which in turn combine less stronglyinto agglomerates. The surface area, as measured by the BET method givesthe best measure of the reinforcing character of different silicas. Forsilicas of interest for the present invention, the surface area shouldbe about 32 m²/g to about 400 m²/g, with the range of about 100 m²/g toabout 250 m²/g being preferred, and the range of about 150 m²/g to about220 m²/g being most preferred. The pH of the silica filler is generallyabout 5.5 to about 7 or slightly over, preferably about 5.5 to about6.8.

Silica can be employed in the amount of about one to about 150 parts byweight per hundred parts of the elastomer (phr), preferably in an amountof about five to about 80 phr and, more preferably, in an amount ofabout 30 to about 80 phr. The useful upper range is limited by the highviscosity imparted by fillers of this type. Some of the commerciallyavailable silicas which can be used include, but are not limited to,Hi-Sil® 190, Hi-Sil® 210, Hi-Sil® 215, Hi-Sil® 233, Hi-Sil® 243, and thelike, produced by PPG Industries (Pittsburgh, Pa.). A number of usefulcommercial grades of different silicas are also available from DegussaCorporation (e.g., VN2, VN3), Rhone Poulenc (e.g., Zeosil® 1165 MP), andJ. M. Huber Corporation.

The elastomers can be compounded with all forms of carbon black in amixture with the silica. The carbon black can be present in amountsranging from about one to about 90 phr, with about five to about 60 phrbeing preferred. The carbon blacks can include any of the commonlyavailable, commercially-produced carbon blacks, but those having asurface area (EMSA) of at least 20 m²/g and, more preferably, at least35 m²/g up to 200 m²/g or higher are preferred. Surface area values usedin this application are determined by ASTM D-1765 using thecetyltrimethyl-ammonium bromide (CTAB) technique. Among the usefulcarbon blacks are furnace black, channel blacks and lamp blacks. Morespecifically, examples of useful carbon blacks include super abrasionfurnace (SAF) blacks, high abrasion furnace (HAF) blacks, fast extrusionfurnace (FEF) blacks, fine furnace (FF) blacks, intermediate superabrasion furnace (ISAF) blacks, semi-reinforcing furnace (SRF) blacks,medium processing channel blacks, hard processing channel blacks andconducting channel blacks. Other carbon blacks which can be utilizedinclude acetylene blacks. A mixture of two or more of the above blackscan be used in preparing the carbon black products of the invention.Typical suitable carbon blacks are N-110, N-220, N-339, N-330, N-351,N-550 and N-660, as designated by ASTM D-1765-82a. The carbon blacksutilized in the preparation of the vulcanizable elastomeric compositionsof the invention can be in pelletized form or an unpelletized flocculentmass. Preferably, for more uniform mixing, unpelletized carbon black ispreferred.

The vulcanizable rubber compositions of the invention can optionallyfurther include a silica coupling agent such as, but not limited to, amercaptosilane, a blocked mercaptosilane, abis(trialkoxysilylorgano)polysulfide, a 3-thiocyanatopropyltrimethoxysilane, silanes that are carried on a filler such as silica,carbon black and the like, or any of the silica coupling a gents thatare known to those of ordinary skill in the rubber compounding art.Exemplary mercaptosilanes include, but are not limited to,1-mercapto-methyltriethoxysilane, 2-mercaptoethyltriethoxysilane,3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldiethoxysilane,2-mercaptoethyltriproxysilane,18-mercaptooctadecyl-diethoxychlorosilane, and the like. Themercaptosilane can be present in the compound in an amount of about0.0001% to about 3% by weight, typically about 0.001% to about 1.5% byweight, and especially about 0.01% to about 1% by weight, based on theweight of the silica. Exemplary blocked mercaptosilanes include, but arenot limited to, octanoyl blocked mercaptans and the like. Exemplarybis(trialkoxysilylorgano)polysulfide silica coupling agents include, butare not limited to, bis(3-triethoxysilyl-propyptetrasulfide (TESPT),which is sold commercially under the tradename Si69 by Degussa Inc., NewYork, N.Y., and bis(3-triethoxysilylpropyl) disulfide (TESPD) or Si75,available from Degussa, or Silquest® A1589, available from Crompton. Thepolysulfide organosilane silica coupling agent can be present in anamount of about 0.01% to about 20% by weight, based on the weight of thesilica, preferably about 0.1% to about 15% by weight, and especiallyabout 1% to about 10%.

Exemplary silica shielding agents suitable for use in the inventioninclude, but are not limited to an alkyl alkoxysilane, analkoxy-modified silsesquioxane (AMS) a mercaptan/alkoxy-modified co-AMS,an amino AMS, an amino/mercaptan co-AMS, a fatty acid ester of ahydrogenated or non-hydrogenated C₅ or C₆ sugar, a polyoxyethylenederivative of a fatty acid ester of a hydrogenated or non-hydrogenatedC₅ or C₆ sugar, and mixtures thereof or a mineral or non-mineraladditional filler, as described in greater detail below. Exemplary fattyacid esters of hydrogenated and non-hydrogenated C₅ and C₆ sugars (e.g.,sorbose, mannose, and arabinose) that are useful as silica dispersingaids include, but are not limited to, the sorbitan oleates, such assorbitan monooleate, dioleate, trioleate and sesquioleate, as well assorbitan esters of laurate, palmitate and stearate fatty acids. Fattyacid esters of hydrogenated and non-hydrogenated C₅ and C₆ sugars arecommercially available from ICI Specialty Chemicals (Wilmington, Del.)under the trade name SPAN®. Representative products include SPAN® 60(sorbitan stearate), SPAN® 80 (sorbitan oleate), and SPAN® 85 (sorbitantrioleate). Other commercially available fatty acid esters of sorbitanare also available, such as the sorbitan monooleates known as Alkamul®SMO; Capmul® O; Glycomul® O; Arlacel® 80; Emsorb® 2500; and S-Maz® 80. Auseful amount of these additional silica dispersing aids when used withthe bis(trialkoxysilylorgano) polysulfide silica coupling agents isabout 0.1% to about 25% by weight based on the weight of the silica,with about 0.5% to about 20% by weight being preferred, and about 1% toabout 15% by weight based on the weight of the silica being morepreferred. In the alkyl alkoxysilane and mercaptosilane embodiment ofthe invention, it may be desirable to use about 0.1% to about 20% byweight of the fatty acid ester based on the weight of the silica. Estersof polyols, including glycols such as polyhydroxy compounds and thelike, in the same quantities, are also useful in all inventionembodiments.

Exemplary polyoxyethylene derivatives of fatty acid esters ofhydrogenated and non-hydrogenated C₅ and C₆ sugars include, but are notlimited to, polysorbates and polyoxyethylene sorbitan esters, which areanalogous to the fatty acid esters of hydrogenated and non-hydrogenatedsugars noted above except that ethylene oxide groups are placed on eachof the hydroxyl groups. Representative examples of polyoxyethylenederivatives of sorbitan include POE® (20) sorbitan monooleate,Polysorbate® 80, Tween® 80, Emsorb® 6900, Liposorb® 0-20, T-Maz® 80, andthe like. The Tween® products are commercially available from ICISpecialty Chemicals. Generally, a useful amount of these optional silicadispersing aids is about 0.1% to about 25% by weight based on the weightof the silica, with about 0.5% to about 20% by weight being preferred,and about 1% to about 15% by weight based on the weight of the silicabeing more preferred.

The silica coupling agents, silica shielding agents and/or other silicadispersing aids and/or other liquid components of the composition can befully or partially supported by the reinforcing filler. The ratio of thecomponent to the reinforcing filler is not critical. If the dispersingaid is a liquid, a suitable ratio of dispersing aid to filler is thatwhich results in a suitably dry material for addition to the elastomer.For example, the ratio can be about 1/99 to about 70/30, about 20/80about 60/40, about 50/50, and the like.

Certain additional fillers can be utilized according to the presentinvention as processing aids, including mineral fillers, such as clay(hydrous aluminum silicate), talc (hydrous magnesium silicate), aluminumhydrate [Al(OH)₃] and mica, as well as non-mineral fillers such as ureaand sodium sulfate. Preferred micas principally contain alumina andsilica, although other known variants are also useful. The foregoingadditional fillers are optional and can be utilized in the amount ofabout 0.5 to about 40 phr, preferably in an amount of about one to about20 phr and, more preferably in an amount of about one to about 10 phr.These additional fillers can also be used as non-reinforcing fillers tosupport the strong organic base catalysts, as well as any of the silicadispersing aids, and silica coupling agents described above. As with thesupport of the silica dispersing aid on the reinforcing filler, asdescribed above, the ratio of dispersing aid to non-reinforcing filleris not critical. For example, the ratio can be about 1/99 to about70/30, about 20/80, about 60/40, about 50/50, and the like, by weight.

The vulcanizable rubber compositions are compounded or blended by usingmixing equipment and procedures conventionally employed in the art, suchas mixing the various vulcanizable polymer(s) with reinforcing fillersand commonly used additive materials such as, but not limited to, curingagents, activators, retarders and accelerators; processing additives,such as oils; resins, including tackifying resins; plasticizers;pigments; additional fillers; fatty acid; zinc oxide; waxes;antioxidants; antiozonants; peptizing agents; and the like. As known tothose skilled in the art, the additives mentioned above are selected andcommonly used in conventional amounts.

Preferably, an initial master batch is prepared that includes the rubbercomponent and the reinforcing fillers, as well as other optionalnon-curative additives, such as processing oil, antioxidants, and thelike. After the master batch is prepared, one or more optional remillstages can follow in which either no ingredients are added to the firstmixture, or the remainder of the non-curing ingredients are added, inorder to reduce the compound viscosity and improve the dispersion of thereinforcing filler. The final step of the mixing process is the additionof vulcanizing agents to the mixture.

According to the embodiments of this invention, the1,2-cyclohexanedicarboxylic acid ester can be added during preparationof the master batch and/or during subsequent stages, and/or the finalstage, and still provide the desired processability of the compound, aswell as the favorable mechanical, dynamic viscoelastic and tensileproperties of the final rubber compound.

The vulcanizable composition can then be processed according to ordinarytire manufacturing techniques. Likewise, the tires are ultimatelyfabricated by using standard rubber curing techniques. For furtherexplanation of rubber compounding and the additives conventionallyemployed, one can refer to The Compounding and Vulcanization of Rubber,by Stevens in Rubber Technology, Second Edition (1973 Van NostrandReibold Company), which is incorporated herein by reference. Thereinforced rubber compounds can be cured in a conventional manner withknown vulcanizing agents at about 0.1 to 10 phr. For a generaldisclosure of suitable vulcanizing agents, one can refer to Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd ed., Wiley Interscience, N.Y.1982, Vol. 20, pp. 365 to 468, particularly Vulcanization Agents andAuxiliary Materials, pp. 390 to 402, or Vulcanization by A. Y. Coran,Encyclopedia of Polymer Science and Engineering, Second Edition (1989John Wiley & Sons, Inc.), both of which are incorporated herein byreference. Vulcanizing agents can be used alone or in combination.Preferably, the rubber compounds are sulfur-vulcanized. Cured orcrosslinked polymers will be referred to as vulcanizates for purposes ofthis disclosure.

The vulcanizable rubber compositions of the present invention can beutilized to form treadstocks for tires. Pneumatic tires can be madeaccording to the constructions disclosed in U.S. Pat. Nos. 5,866,171;5,876,527; 5,931,211; and 5,971,046, the disclosures of which areincorporated herein by reference. The composition can also be used toform other elastomeric tire components such as subtreads, blacksidewalls, body ply skims, bead fillers and the like.

EXAMPLES

The following examples illustrate a comparison of the physicalproperties of rubber compounds containing a 1,2-cyclohexanedicarboxylicacid, diisononyl ester plasticizer (Hexamoll™ DINCH from BASF,abbreviated to Hexamoll™ in each of the examples), with the physicalproperties of rubber compounds containing dioctylphthalate (DOP) andtire components containing them. The examples include a carbon blackformulation and two formulations containing both silica and carbonblack. However, the examples are not intended to be limiting, as otherrubber compounds, including different compounding formulations anddifferent cyclohexanepolycarboxylic acid esters can be employed, asdetermined by those skilled in the art without departing from the scopeof the invention herein disclosed and claimed.

Example 1 Compounding of Carbon Black-filled Rubber

The plasticizer, Hexamoll™ was used in comparison with dioctylphthalate(DOP) in the otherwise identical rubber compositions shown in Table 1.Each of the rubber compounds was prepared in two stages, namely aninitial masterbatch, and a final stage. The initial stage was mixed in a65 g Banbury mixer operating at 60 RPM and 130° C. First, the elastomerwas placed in the mixer and, after 30 seconds, the remaining ingredientsexcept stearic acid were added. At 2.5 minutes, stearic acid was added.From 4 5 minutes, the rotor speed was increased to 90 RPM and theingredients were mixed for 6 minutes. At the end of this mixing, themixing temperature was approximately 147° C. to 156° C. The samples weretransferred to a mill operating at a temperature of 60° C., where theywere sheeted and subsequently cooled to room temperature.

The final stage was mixed by adding the material from the initial stageand the curative materials to the mixer simultaneously. The initialmixer temperature was 60° C. and it was operating at 40 RPM. The finalmaterial was removed from the mixer after 3 minutes when the materialtemperature was between 90° C. and 95° C. The formulations were thenprepared into test specimens and cured within a closed cavity mold underpressure for 15 minutes at 171° C. The test specimens were thensubjected to various physical tests, and the results of these tests arereported in Tables 2-4.

Example 2 Processing Evaluation of Carbon Black-filled Rubber

1. The Green Stock Mooney Viscosity and Curing Characteristics

The processing of the green stocks (i.e., the stock obtained after thefinal stage, prior to curing) was characterized as to Mooney viscosityand cure characteristics. In particular, the stock 2 containingHexamoll™ was evaluated and compared with the stock 1 containing DOP byexamining the compound Mooney viscosity and scorch characteristics. TheMooney viscosity measurement was conducted at 130° C. using a largerotor. The Mooney viscosity was recorded as the torque when the rotorwas rotated for 4 minutes. The samples were preheated at 130° C. for oneminute before the rotor was started. A Monsanto Rheometer MD2000 wasused to characterize the stock curing process. The frequency was 1.67 Hzand the strain was 7% at 160° C. The T90 obtained from thesemeasurements represents the time when the torque rose to 90% of thetotal torque increase during the curing process. This measurement wasused to predict the curing rate during the curing process. The resultsof the compound Mooney (ML₁₊₄) and curing characteristics are shown inTable 2 and illustrate that the Hexamoll™-containing stock 2 had resultsthat were comparable to those obtained with the DOP-containing stock 1.

TABLE 1 Stock No. DOP Stock 1 Hexamoll ™ Stock 2 (phr) (phr) MasterbatchSolution SBR, tin coupled* 100 100 Carbon Black 50 50 Dioctylphthalate10 0 Hexamoll ™ 0 10 Wax 2 2 Antioxidant** 1 1 Stearic Acid 2.0 2.0Oil*** 10 10 Final Zinc Oxide 2.5 2.5 Sulfur 1.5 1.5 Diphenyl guanidine1.3 1.3 N-tert-butyl-2- 0.5 0.5 benzothiazolesulfenamide2,2′dithiobis(benzothiazole) 0.5 0.5 *SBR = 20% styrene, 55% vinyl, ML₄= 72 **Santoflex ® 6PPD ***Hyprene BO300

TABLE 2 Stock No. DOP Control Hexamoll ™ Stock 1 Stock 2 ML₁₊₄ @ 130° C.33.0 32.8 MH-ML (kg-cm) 12.64 12.48 T90 (min) 7.50 8.042. Dynamic Viscoelastic Mechanical Properties

The dynamic viscoelastic mechanical properties were obtained fromtemperature sweep tests conducted with a frequency of 31.4 rad/sec using0.2% strain for temperatures ranging from −100° C. to −10° C. and 2%strain for temperatures ranging from −10° C. to 100° C. The 50° C. tan δdata were also obtained from strain sweep measurements at a strain levelof 5%. A frequency of 31.4 rad/sec was used for the strain sweep whichwas conducted at 60° C. with strain sweeping from 0.25% to 14.75%. Thedegree of filler flocculation after compounding (the Payne effect, ΔG′)was estimated by subtracting the storage modulus G′ at 14.25% strainfrom the G′ at 0.25% strain. The value of tan δ at 0° C. can be used topredict the tire wet traction, and the value of the tan δ at 60° C. canbe used to predict the rolling resistance properties of the tires. TheG′ at −30° C. is a predictor of tire ice and snow traction.

Other viscoelastic properties were measured by using the dynamiccompression test. The sample geometry used for dynamic compression testwas a cylindrical button 9.5 mm in diameter and 15.6 mm in length. Thesample was compressed under a static load of 2 kg before testing. Afterit reached an equilibrium state, the test began with a dynamiccompression load of 1.25 Kg at a frequency of 1 Hz. The sample was thendynamically compressed and then extended and the resultant displacementand hysteresis (tan δ) were recorded. The viscoelastic properties areshown in Table 3 for the two rubber stocks.

TABLE 3 Stock No. DOP Stock 1 Hexamoll ™ Stock 2 G′, −30° C., 0.2%strain, MPa 767 761 tan δ, 0° C., 10 Hz, 0.2% strain 0.535 0.518Dynastat tan δ, 0° C. 0.549 0.576 tan δ, 60° C., 10 Hz, 2% strain 0.1370.147 ΔG′, 60° C., 10 Hz, MPa 0.731 0.732 tan δ, 60° C., 5% strain 0.1310.136 Dynastat tan δ, 60° C. 0.126 0.1233. Tensile Mechanical Properties

The tensile mechanical properties were measured using the standardprocedure described in ASTM-D 412 at 25° C. Test specimens aremicrodumbell samples. The symbols M50, M200 and M300 correspond tomodulus at 50%, 200% and 300% elongation respectively. The symbols T_(b)and E_(b) refer to the tensile at break and percent elongation at break.Toughness is a measure of the area under the stress-strain curve. Thetensile properties for the two stocks are shown in Table 4.

TABLE 4 Stock No. DOP Stock 1 Hexamoll ™ Stock 2 Dumbell Tensile @ 25°C. M50 (MPa) 1.17 1.14 M300 (MPa) 12.0 12.2 T_(b) (MPa) 12.8 18.0 E_(b)% 314 396 Toughness (MPa) 15.9 29.1 Dumbell Tensile@ 100° C. M50 (MPa)1.07 1.05 M300 (MPa) 5.74 5.72 T_(b) (MPa) 6.80 6.50 E_(b) % 222 217Toughness (MPa) 6.35 5.89

As illustrated by the data in Tables 2 to 4, the physical properties ofthe carbon black compounded rubbers are essentially the same, whetherDOP or Hexamoll™ is used as the plasticizer.

Example 3 Compounding of Carbon Black and Silica Filled Rubber

The plasticizer, Hexamoll™ was used in comparison with dioctylphthalate(DOP) in the otherwise identical rubber compositions shown in Table 5.Each of the rubber compounds was prepared in three stages, namely aninitial masterbatch, second masterbatch and a final stage. The initialstage was mixed in a 300 g Banbury mixer operating at 50 RPM and 90° C.First, the elastomer was placed in the mixer and, after 30 seconds, theremaining ingredients except stearic acid were added. At 2.5 minutes,stearic acid was added. From 4 5 minutes, the rotor speed was increasedto 90 RPM and the ingredients were mixed for 6 minutes. At the end ofthis mixing, the mixing temperature was approximately 147° C. to 156° C.The samples were transferred to a mill operating at a temperature of 60°C., where they were sheeted and subsequently cooled to room temperature.

The second masterbatch stage was mixed by adding the material from theinitial stage and the curative materials to the mixer simultaneously.The initial mixer temperature was 100° C. and it was operating at 50RPM. The final material was removed from the mixer after 3 minutes whenthe material temperature was between 140° C. and 150° C. The sampleswere transferred to a mill operating at a temperature of 60° C., wherethey were sheeted and subsequently cooled to room temperature.

The final stage was mixed by adding the material from the initial stageand the curative materials to the mixer simultaneously. The initialmixer temperature was 60° C. and it was operating at 40 RPM. The finalmaterial was removed from the mixer after 3 minutes when the materialtemperature was between 100° C. and 110° C. The formulations were thenprepared into test specimens and cured within a closed cavity mold underpressure for 15 minutes at 171° C. The test specimens were thensubjected to various physical tests, and the results of these tests arereported in Tables 6 through 8.

TABLE 5 Stock No. DOP Stock 3 Hexamoll ™ Stock 4 (phr) (phr) MasterbatchSolution SBR, tin coupled* 100 100 Carbon Black 27 27 Silica 24.5 24.5Dioctylphthalate 5 0 Hexamoll ™ 0 5 Wax 2 2 Antioxidant** 0.95 0.95Stearic Acid 2.0 2.0 Oil*** 10 10 Second Masterbatch Silica 2.5 2.5 S2Silane† 2.5 2.5 Final Zinc Oxide 2.5 2.5 Sulfur 1.5 1.5 Diphenylguanidine 0.7 0.7 N-tert-butyl-2- 0.6 0.6 benzothiazolesulfenamide2,2′dithiobis(benzothiazole) 0.5 0.5 *SBR = 20% styrene, 55% vinyl, ML₄= 72 **Santoflex ® 6PPD ***Hyprene BO300 †bis(3-triethoxysilylpropyl)disulfide

Example 4 Processing Evaluation of Carbon Black and Silica Filled Rubber

The green stock Mooney viscosity and curing characteristics, and thedynamic viscoelastic mechanical properties and tensile mechanicalproperties of the cured stocks were measured as described in Example 2.In particular, the stock 4 containing Hexamoll™ was evaluated andcompared with the stock 3 containing DOP. The results are illustrated inTables 6, 7 and 8, respectively.

TABLE 6 Stock No. DOP Control Hexamoll ™ Stock 3 Stock 4 ML₁₊₄ @ 130° C.42.2 43.7 MH-ML (kg-cm) 17.85 17.75 T90 (min) 11.7 11.8

TABLE 7 Stock No. DOP Stock 3 Hexamoll ™ Stock 4 G′, −30° C., 0.2%strain, MPa 1001 1000 tan δ, 0° C., 10 Hz, 0.2% strain 0.530 0.533 tanδ, 60° C., 10 Hz, 2% strain 0.138 0.136 ΔG′, 60° C., 10 Hz, MPa 1.181.11 tan δ, 60° C., 5% strain 0.124 0.121 Dynastat tan δ, 60° C. 0.1140.113

TABLE 8 Stock No. DOP Stock 3 Hexamoll ™ Stock 4 Dumbell Tensile @ 25°C. M50 (MPa) 1.52 1.56 M300 (MPa) 10.8 10.4 T_(b) (MPa) 20.1 18.7 E_(b)% 488 473 Toughness (MPa) 43.7 39.4 Dumbell Tensile@ 100° C. M50 (MPa)1.52 1.43 M200 (MPa) 6.3 6.0 T_(b) (MPa) 9.1 9.0 E_(b) % 272 282Toughness (MPa) 11.6 11.9

As illustrated by the data in Tables 6 through 8, the physicalproperties of the carbon black and silica compounded rubbers areessentially the same, whether DOP or Hexamoll™ is used as theplasticizer.

Therefore, it has been demonstrated that environmentally non-toxiccyclohexanepolycarboxylic acid esters, particularly1,2-cyclohexanedicarboxylic acid esters and the like, are useful asplasticizers in rubber compounds. Further, such1,2-cyclohexanedicarboxylic acid esters provide equivalent rubberproperties to those provided by the phthalic acid diesters, includingdesirable wet traction and snow traction, while maintaining otheradvantageous physical properties of the rubbers.

While the invention has been described herein with reference to thepreferred embodiments, it is to be understood that it is not intended tolimit the invention to the specific forms disclosed. On the contrary, itis intended that the invention cover all modifications and alternativeforms falling within the scope of the appended claims.

We claim:
 1. A vulcanizable rubber composition comprising: (a) avulcanizable elastomer; (b) a reinforcing filler comprising a selectionfrom the group consisting of silica, carbon black, and mixtures thereof;(c) a plasticizer comprising a 1,2-cyclohexanedicarboxylic acid ester;and (d) a vulcanization agent.
 2. The vulcanizable rubber composition ofclaim 1, wherein the vulcanizable elastomer is selected from the groupconsisting of: homopolymers of a conjugated diene monomer, andcopolymers and terpolymers of the conjugated diene monomers withmonovinyl aromatic monomers and trienes.
 3. The vulcanizable rubbercomposition of claim 1, wherein the 1,2-cyclohexane-dicarboxylic acidester is present in the composition in an amount of about 0.01 to about100 phr.
 4. The vulcanizable rubber composition of claim 1, wherein theamount of the 1,2-cyclohexanedicarboxylic acid ester is about 1 to about40 phr.
 5. The vulcanizable rubber composition of claim 1, wherein theamount of the 1,2-cyclohexanedicarboxylic acid ester is about 0.01 toabout 20 phr.
 6. The vulcanizable rubber composition of claim 1, whereinthe amount of the 1,2-cyclohexanedicarboxylic acid ester is about 5 toabout 40 phr.
 7. The vulcanizable rubber composition of claim 1, whereinthe 1,2-cyclohexane-dicarboxylic acid ester is selected from the groupconsisting of 1,2-diisobutyl-cyclohexanedicarboxylic acid ester;1,2-di-(2-ethylhexyl)-cyclohexanedicarboxylic acid ester;1,2-diisononylcyclohexanedicarboxylic acid ester;1,2-di-C₉-cyclohexane-dicarboxylic acid ester;1,2-diisopentylcyclohexanedicarboxylic acid;1,2-diisoheptyl-cyclohexanedicarboxylic acid;1,2-diisodecylcyclohexane-dicarboxylic acid;1,2-di-C₇₋₁₁-cyclohexanedicarboxylic acid;1,2-di-C₉₋₁₁-cyclohexanedicarboxylic acid;1,2-di-C₇₋₉-cyclohexanedicarboxylic acid; and mixtures thereof.
 8. Thevulcanizable rubber composition of claim 1, wherein the1,2-cyclohexane-dicarboxylic acid ester comprises1,2-diisononylcyclohexanedicarboxylic acid ester.
 9. The vulcanizablerubber composition of claim 1, wherein the vulcanizable elastomer isselected from the group consisting of natural rubber, syntheticpolyisoprene rubber, styrene/butadiene rubber (SBR), polybutadiene,butyl rubber, neoprene, ethylene/propylene rubber, ethylene/propylenediene rubber (EPDM), acrylonitrile/butadiene rubber (NBR), siliconerubber, fluoroelastomers, ethylene acrylic rubber, ethylene vinylacetate copolymers (EVA), epichlorohydrin rubbers, chlorinatedpolyethylene rubber, chlorosulfonated polyethylene, rubbers,hydrogenated nitrile rubber, and tetrafluoroethylene/propylene rubber.10. The vulcanizable rubber composition of claim 1, wherein thevulcanizable rubber composition is not an adhesive.
 11. The vulcanizablerubber composition of claim 1, wherein the vulcanizable elastomercomprises styrene/butadiene rubber (SBR) and the amount of the1,2-cyclohexanedicarboxylic acid ester is about 5 to about 40 phr.
 12. Avulcanizable rubber composition comprising: (a) a vulcanizable elastomerselected from the group consisting of natural rubber, syntheticpolyisoprene rubber, styrene/butadiene rubber (SBR), polybutadiene,butyl rubber, ethylene/propylene rubber, ethylene/propylene diene rubber(EPDM), acrylonitrile/butadiene rubber (NBR), and hydrogenated nitrilerubber; (b) a reinforcing filler comprising silica, carbon black, ormixtures thereof; (c) about 1 to about 40 phr of a1,2-cyclohexanedicarboxylic acid ester, selected from the groupconsisting of 1,2-diisobutyl-cyclohexanedicarboxylic acid ester;1,2-di-(2-ethylhexyl)-cyclohexanedicarboxylic acid ester;1,2-diisononylcyclohexanedicarboxylic acid ester;1,2-di-C₉-cyclohexane-dicarboxylic acid ester;1,2-diisopentylcyclohexanedicarboxylic acid;1,2-diisoheptyl-cyclohexanedicarboxylic acid;1,2-diisodecylcyclohexane-dicarboxylic acid;1,2-di-C₇₋₁₁-cyclohexanedicarboxylic acid;1,2-di-C₉₋₁₁-cyclohexanedicarboxylic acid;1,2-di-C₇₋₉-cyclohexanedicarboxylic acid; and mixtures thereof; and (d)a sulfur vulcanization agent.